In image reading apparatuses such as scanners and image forming apparatuses such as copiers, an exposure lamp is made to reciprocate along a document plate and an original document on the document plate is exposed by the exposure lamp at the time of moving forward. A CCD sensor which is a photoelectric conversion device receives light reflected from the original document in the exposure. The CCD sensor linearly scans the received light in a direction perpendicular to the moving direction of the exposure lamp and repeats the linear scanning with the forward movement of the exposure lamp. The direction of the linear scanning of the CCD sensor is the main scanning direction. The forward moving direction of the exposure lamp is the sub scanning direction.
In a CIS type in which the CCD sensor is made to reciprocate along the document plate along with the exposure lamp, the length of the CCD sensor in the linear scanning direction is almost equal to the width of the document plate. The CCD sensor includes plural linear sensors connected in series in a line shape and outputs an image signal with a voltage level corresponding to the amount of received light by performing a linear scanning operation using the reflected light received from the original document.
The outputs at connecting positions among the linear sensors of the CCD sensor may have a difference. This difference in outputs appears as a concentration spot at the time of forming an image.